1. Field of the Invention
The present invention relates to shields used for shielding tubes in power or recovery boilers. More specifically, the invention relates to tube shields which are secured to the tubes using a tongue and locking block assembly.
2. Related Art
As described in U.S. Pat. No. 5,220,957 to Hance, which is incorporated herein by reference in its entirety, three common methods of installing tube shields on boiler tubes are the U-bolt system (shown in FIGS. 1 and 4 of Hance); the snap system (shown in FIGS. 2 and 5 of Hance); and the welded U-strap system (shown in FIGS. 3 and 6 of Hance). As is noted by Hance, once the tube shields of these systems have been installed on the tubes and the boiler is placed in service, they tend to loosen, rotate, and slip axially due to expansion and contraction resulting from temperature fluctuations. A fourth method is described in U.S. Pat. No. 4,619,314 to Shimoda and employs pins which extend through projections on the sides of the tube shields.
Hance's invention is directed to a lug and slot installation which provides improved protection against loosening, rotation, and axial slippage of the tube shield in comparison to the U-bolt, snap, and welded U-strap systems; and a simpler but equally effective construction in comparison to Shimoda's system. It also provides the advantage of being usable in both horizontal and vertical orientations. However, Hance's lug and slot installation has to pass inspection before installation.
Specifically, the lug in Hance's installation is secured to the existing boiler tube by welding. This weld has to be tested by non-destructive testing (NDT) personnel to assure that it is acceptable, and that no leakage occurs. If the weld is not acceptable, then repairs have to be made and the weld tested again. The tube shield itself cannot be installed until this procedure has been accomplished, due to the fact that the tube shield, once installed, will restrict any required repair of the weld on the lug.
Every time testing of the welds is required, all personnel must be evacuated from the inside of the boiler, to allow the NDT personnel to work. This procedure is extremely costly.
Only after all lugs have been installed, tested, and approved or repaired can the tube shield be installed. Two workers are required for installation of each tube shield, because the shield must be held in place by one worker wile the other one welds at least one of the retainer straps in place. As the tube shields are installed in locations in the boilers above where other people are working, there is the potential for serious injury if one of the shields were to fall.
Further, the slot openings of Hance's lug and slot installation provide a place where build-up of residue occurs, and leave some of the underlying tube exposed and subject to wear and erosion. It is to the solution of these and the above problems to which the present invention is directed.